Troubleshooting Serial Connections

To effectively identify and troubleshoot any problem, including those with your serial connections, you must have the key information about the devices or services you are troubleshooting. There are some show and debug commands you can use to identify configuration or flow issues with serial connections.

Showing

The show command displays the configuration and status of your serial ports. In the following code, the items that stand out the most are the line protocols that are reported as down, meaning that the physical connection to the device at the other end of the connection is not established, as well as the encapsulation protocol that is in use, such as HDLC or PPP.

Debugging

After you know how to view current configuration information, take a look at the active connection and data going through the connection by using the debug command. You can choose from the following debug options. which focus on the interface option.

When you see the show command named Showing in the preceding section, you start with an issue with clock speed not being properly set on the DCE side of the connection. To see and diagnose a problem, the problem was re-created the problem so that you can see what it would look like when troubleshooting with the debug command.

When you have an issue with your connection, you can take a look at the information the debug command gives you about the serial interface. After the code example, some information that you should be able to deduce is provided.

There is an issue with communications, as shown by the asserting, deasserting, and reset commands. These are typically never good messages to see. Specifically, there is a problem in confirming serial communication commands, such as Ready to Send (RTS) and Clear to Send (CTS).

There is an issue with the HDLC communication, and after several attempts, it gives up, identified by the Cable is unplugged statement.

Another issue that shows up in the output is related to the lines that include the words mineseen and yourseen. They refer to the number of data control packets that have been sent over the connection, which are used only to monitor the connection.

When the router sends data over an HDLC connection, it tracks a sequential number to ensure that all the data gets through. The number on Router1 is what Router1 calls mineseen, whereas Router1 call the sequence number reported by the other router yourseen.

Although HDLC is saying that the cable may be unplugged, you already know that the problem lies in the clock speed, which to Router1 looks the same because it cannot start the connection properly.

With the clock speed correctly set, the debug data looks like the following example. Notice the mineseen and yourseen data and that the line is up. The difference in the numbers occurs because Router2 has been sending data control packets that this router has never seen, because the interface was brought up later.